During transmission of electrical signals a variety of disturbances such, for example, as background noise, crackle, spluttering and contact chatter are apt to occur in the transmission path. Usual practice is, in general, to seek to get rid of such disturbances by providing at the receiving end of the transmission path suitable frequency and/or amplitude sensitive analog filter arrangements which attenuate the disturbances so that disturbances of frequencies and/or amplitudes lying outside the frequency and/or amplitude ranges of the desired useful signals are reduced or eliminated. However, in practice very brief but high amplitude disturbances, strong enough to be able to lead to erroneous evaluation of the desired signals still reach the receiving equipment and are not eliminated or sufficiently reduced by the filter arrangements referred to. Moreover, the provision of such filter arrangements in the transmission lines of data transmission installations may be very costly, especially in the case of data transmission systems in which the range of the transmission speeds which must be handled is very large, e.g. between 50 and 10,000 baud. Moreover, integrated analog filters have the defect that while they reduce the amplitudes of disturbances they extend them in time.
It is known to reduce disturbance signals occurring within the useful transmission band width by using them to produce control signals by means of which they are combined in opposition with signals produced when they occur and for the duration of their occurrence, so that the signals which are combined are mutually cancelling. However, this expedient can be usefully applied only for the elimination of disturbance signals which arise during pauses between desired signals. This, of course, is because this expedient of getting rid of disturbances by what is essentially a self cancelling signal method produces gaps in the signals and if such cancellation is effected during the presence of desired signals, information is caused to be lost. If the proportion of information lost in this way exceeds a certain value, the legibility of the desired transmitted signals is reduced so much that the information they contain is no longer utilisable. There therefore still remains the problem of securing adequate elimination of disturbance signals without unacceptable interference with no loss of desired useful signals.
A noise elimination circuit arrangement for binary information signals and which is designed to solve the aforesaid problem within certain limits has already been proposed. This circuit arrangement (which will be found described in German Auslegeschrift DT-AS 2165461) consists of two bistable circuits and a number of NAND gates. Signals containing disturbance components are fed in to the circuit arrangement and there interlinked with signals of a cyclic nature which are obtained from a specially provided generator. The arrangement is such that the desired useful signals, with the disturbance signals removed at least to a large extent, are recovered at the output of the circuit arrangement but are retarded by two complete cycles in relation to the signals fed in. This is a disadvantage. Another disadvantage is the need to provide a generator which produces cyclic signals of a fixed predetermined period which must be such that twice its value must be both greater than the expected duration of a disturbance signal and less than the duration of a desired useful signal. Because of this the said cyclic signal generator must be of high accuracy and precision of design and operation. Furthermore there is the defect that the operation of the circuit arrangement depends on control by outside signals and if these fail to appear elimination of the disturbance signals does not occur. Finally the correct functioning of the circuit arrangement in question is in large measure dependent on the relative position in time of the disturbance signals with respect to the cyclic signals, and when any material deviation from a given relationship in time occurs, the desired useful signals may become distorted.
The present invention seeks to avoid the disadvantages of the various known arrangements above mentioned and to provide improved circuit arrangements of a relatively simple nature which will provide adequately effective suppression of disturbance signals without unacceptable distortion or loss of useful desired signals; which do not involve the provision of any special cyclic signal generator; and which function only when signal changes occur, so that the need for continuous actuation by special signals derived from an outside source is eliminated.
According to the present invention, there is provided a pulse suppression circuit arrangement, suitable for use in a data transmission system, for suppressing, in dependence on the length thereof, noise, disturbance or other undesired signal pulses of lengths outside a range of lengths in which useful signal pulses lie wherein a train of output signal pulses, freed of superimposed errors caused by said undesired signal pulses, is obtained by at least one process of coupling incoming signal pulses with generated pulses produced when said incoming signal pulses occur, each such coupling process consisting in coupling incoming signals or signals derived therefrom with the generated pulse output from triggerable pulse generator means triggered by signal pulse edges of one polarity and the time constant of which is less than the useful signal pulse length and greater than undesired signal pulse length and again effecting such coupling under the control of incoming signal edges of the other polarity.
Preferably the triggerable pulse generator means are of the monostable type. As will be seen, if the generator means of this type is employed a train of input signals (or a train corresponding therewith) is at least once interlinked with the output of a monostable circuit actuated by signal edges of one polarity and having a time constant less than the duration of useful signals and longer than the duration of pulses to be suppressed, a further interlinking, while regulation is effected by signal edges of the other second polarity, being effected.
In this way a function which suppresses the undesired pulses is derived from the respective course of the signal itself by logic interlinking of the signals with reference times. The invention thus lends itself to embodiment in relatively inexpensive apparatus of relatively simple circuitry. The operation is reliable and satisfactory for each undesired pulse is suppressed during its transit time and only during this time. Because a monostable circuit or a similar triggerable circuit of like properties controls this suppression, the useful signals are extended only by the transit time of the monostable circuit or the like and this extension is compensated for by the further interlinking which ensues as a result of actuating by signal pulse edges of opposite polarity to those used in connection with the first interlinking so that compensating pulse shortening of the useful pulses occurs. Thus, the overall result is merely a displacement in time of the useful signals by an amount determined by the time constants of the monostable circuits used. The noise suppression operates entirely asynchronously and independently of externally produced cyclic signals, so that it does not produce additional distortions. The criterion for ensuring the described process is always the appearance of a leading edge of a pulse.
The invention may be advantageously employed in the receiving channels of data signal transmission systems for eliminating noise and disturbance produced as a result of signal amplitude limitation or digitalisation. In signal transmission systems using translators (modulators) the elimination of noise and disturbance can be effected after demodulation.
The invention lends itself not only to embodiment in simple circuitry but also to the use integrated circuit techniques in such circuitry. Thus, for example, embodiments using series inverters parallel fed with a monostable circuit (such embodiments will be described later) with the monostable circuit (unstable state) output and the output of the second inverter fed to the respective inputs of a NAND gate which furnishes a train of output signals which is used for the second interlinking operation, can be constructed from two essentially similar and simple circuit arrangements in series, both of which lend themselves to integrated circuit construction. Similar advantages of simplicity and easy manufacture consequent upon the use of a plurality of essentially simple and similar circuit arrangements to constitute an embodiment apply to other circuit arrangements and equipments embodying the invention and later to be described herein.
As will be seen later the invention may be used to suppress pulses other than noise or disturbance pulses and notably to suppress pulses transmitted at transmission speeds outside a specified speed range. It may therefore be used to provide what may be termed a transmission speed "filter" to secure accurate separation of data signals which are to be transmitted or not transmitted respectively in terms of their transmission speed. For example, during base band transmission the requirement of an upper transmission speed limit in one transmission channel can be easily satisfied. Such a "filter" if incorporated at the transmitting end reliably prevents an increase of transmission speed beyond a predetermined value. As will be seen later the limits of the "pass speed range" of such a "filter" can be made easily and independently adjustable. High speed pass "filters," low speed pass "filters" and band speed pass "filters" (to use terms analogous to high pass, low pass and band pass filters) are all easily obtainable in accordance with specified requirements.